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  • 1
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    Mount Etna eruptions and damaging earthquakes: new insights (2007)
    Details
    Publication Date: 2017-04-04
    Description: Between 26 and 29 October 2002, the Mt. Etna area was struck by a seismic swarm that counted some hundred events some of them well felt by the population. A first group of epicenters is located in the E-NE part of the volcano, reaching a maximum local magnitude of 4.6 on October 27 at 02:50 UT. A second group, occurring since October 28, is located in the SE flank close to the town of Zafferana Etnea and culminated with the ML 4.4 earthquake of October 29 at 10:02 UT. The seismic swarm was concomitant with the opening of a system of fractures in the eastern part of the volcano and the reactivation of the eruptive fracture system originated by the 2001 eruption. The most damaging event was the ML 4.4 earthquake of October 29, which occurred in a densely urbanized territory and attained intensity VIII on the European Macroseismic Scale (EMS). In spite of its small local magnitude, the shock caused significant damage to many buildings including reinforced concrete ones in an area that extends for about 4 km in a NNW-SSE direction and is centered around the villages of S. Venerina and Guardia. A long system of surface fractures originated along the damaged area. Other similar and even stronger events of the 2002 swarm in the northern zone did not cause diffuse damage since they occurred in a not densely urbanized area; however, they were highly destructive for the few sparse buildings close to the epicenters. The sequence was well recorded by the Broad Band Station of Antillo (Mednet Network), the most energetic events also triggered the Strong Motion Stations of Bronte (BRNT) and Catania (CATA). The latest part of the sequence (Dec. 2003 – Jan. 2004) was recorded by the local micronetwork installed in Santa Venerina (SVN). Two of these stations were also operating in Santa Venerina from Nov. 2003 to Aug. 2004. Moreover, data from a broad band station installed in Catania are available from Mar. 2001 to Dec. 2001.
    Description: Unpublished
    Description: Quito (Ecuador)
    Description: open
    Keywords: Magnitude, Low frequency, Damage ; 04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Poster session
    Format: 4002816 bytes
    Format: application/vnd.ms-powerpoint
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  • 2
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    The magnitude of damaging volcanic earthquakes of Mt. Etna: are the commonly used scales adequate? (2007)
    Details
    Publication Date: 2017-04-04
    Description: On October 2002 a seismic swarm occurred on the eastern flank of Mt. Etna. One of the strongest events caused severe damage, up to EMS intensity of VIII that contrasts with its local magnitude of 4.4. The occurrence of significant damage at such small magnitude is repeatedly observed in the Mt. Etna area and is traditionally attributed to the shallow source of volcanic earthquakes. Strong-motion accelerograms and broad-band seismograms recorded during the swarm demonstrate that there is a more cogent cause for the severe damage, i.e. an anomalously strong low-frequency (0.1 〈 f 〈 1 Hz) radiation deviating from the conventional Brune (1970) spectral scaling. Therefore, these earthquakes cause unexpectedly large ground displacements and long ( 20 sec) durations of shaking. The integration of digital accelerograms recorded on October 2002 yields a maximum peak ground displacement as large as 1.8 cm at a distances of 18 km, out of the largest damage zone. Based on the sharp local attenuation of ground motion amplitudes observed during the Mt. Etna earthquakes, we infer that displacements near the epicentres can have attained 10 cm. So large displacements are consistent with the maximum observed damage. Moreover, the frequency cutoff below 1.25 Hz in the Wood-Anderson response attenuates the peak-to-peak amplitudes used to assess local magnitudes. This instrumental deamplification at low frequency yields underestimated values of local magnitude that are not representative of the real ground shaking. Since a prompt, correct magnitude (and potential damage) assessment is crucial for efficient Civil Protection actions, a procedure is proposed which, in near-real-time, can be successful in identifying potentially damaging earthquakes of Mt. Etna through the computation of response spectra. The procedure provides a magnitude value that is derived on a statistical basis from the Housner (1952) spectral intensity computed in the low-frequency band. This parameter is a suitable near-real-time indicator of large earthquake-induced building shaking and could also be applied for a preliminary determination of the epicentral macroseismic intensity of volcanic events of Mt. Etna through consolidated relationships established for tectonic earthquakes in Italy.
    Description: Submitted
    Description: open
    Keywords: Magnitude, Low frequency, Damage ; 04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: manuscript
    Format: 623010 bytes
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  • 3
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    Evidence for ground motion polarization on fault zones of mt. etna volcano (2008)
    AGU
    Details
    Publication Date: 2017-04-04
    Description: During local and regional earthquakes, an evident amplification of horizontal ground motion is observed at two seismological stations near the Tremestieri fault, on the southeastern flank of Mt. Etna volcano. Rotated-component spectral ratios show a narrow spectral peak around 4-Hz along a N40°E direction. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal ground motion in the fault zone. The effect is evident during the entire seismogram and independent of source backazimuth as well as distance and depth of earthquakes. The same polarization is observed in ambient noise as well. This consistency allowed us to use microtremors for checking ground motion polarization along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area, interesting a broad frequency band. To check whether this ground motion property is recurrent and understand a possible relationship with fault strike, faulting style, or orientation of fractures, ambient noise was recorded on other mapped faults of the Mt. Etna area, the Moscarello, Acicatena and Pernicana faults. The latter, in particular, is characterized by different strike and faulting style. A systematic tendency of ambient noise to be polarized is found in all of the faults. A picture emerges where normal faults of the eastern flank show a E-W to NE-SW polarization that changes on the Pernicana fault, which develops approximately E-W and is characterized by a prevailing NW-SE to NS polarization. Directions of polarization were never parallel to the fault strike. Moreover, polarization persists too far away from the fault trace, excluding an effect limited to a narrow low velocity zone hosted between harder wall rocks. Both these observations rule out an interpretation in terms of fault-trapped waves. The cause of observed polarizations will be the subject of future studies. However, the consistency with recent results of velocity anisotropy in a part of the investigated area suggests a possible role of attenuation anisotropy on horizontal amplitude variations versus azimuth.
    Description: Published
    Description: B10306
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: open
    Keywords: polarization ; fault zones ; Etna volcano ; microtremors ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion ; 04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring ; 04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology ; 04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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